Effect  of  Ignition  on  the 
Solubility  of  Soil  Phosphate 

C,  B.  Lipman 


C-NRLF 


B  3M  MflD 


Main  Lib.  AGRIC.   DEPT. 


'^MPhm  RYTs 


o 


The  Effect   of  Ignition  on  the  Solu- 
bility of  Soil  Phosphate. 


;AGRICl)LH'«-l  I 
UBRAF 

UNIVERb 


CALIFORNIA. 


By  ,Chas.  6.  Lipmaa. 


iMaiti   I.ih. 
Acyric,    Dept 


[Reprinted  from   the  Journal    of   Industrial   and  Engineering  Ch 
Vol.  4.     No.  9.     September,  1912]. 


I  AbHIUULlL 
LIBRARY, 

UNIVERSJ 

raistry,  qj* 

CALIFORNIA 


THE  EFFECT  OF  IGNITION  ON  THE  SOLUBILITY  OF  SOIL 
PHOSPHATES. 

By  Chas.  B.  Lipman. 
Received  July  31,  1912. 

In  common  with  other  soil  investigators,  the  writer 
has  taken  occasion  to  point  out  recently  the  dangers 
involved  in  applying  data  obtained  with  solution 
cultures  of  plants  to  soil  problems  with  the  same 
plants.  Time  and  again  results  have  been  obtained 
in  solution  cultures  of  both  plants  and  bacteria  which 
when  compared  with  tests  of  the  same  organisms  on 
soils,  both  sterile  and  unsterile,  were  found  to  be  widely 
divergent  and  discordant.  I  have  also  found  the  same 
to  be  true  regarding  the  purely  chemical  changes  of 
the  same  materials  in  minerals  and  in  soils  as  brought 
about  by  the  laboratory  treatment  involved  in  their 
quantitative  determination.  My  remarks  apply  par- 
ticularly to  the  quantitative  determinations  of  phos- 
phates in  soil ;  I  employ  for  this  paper  the  same  title 
that  Fraps^  used  for  his  recently,  to  call  attention  to 
the  differences  effected  in  the  degree  of  solubility 
of  phosphates  in  phosphate  minerals  and  in  soils, 
by  ignition.  In  the  work  just  cited,  Fraps  has  shown 
that  the  ignition  of  phosphate  minerals,  including 
wavellite,  dufrenite  and  variscite,  for  lo  minutes  at 
a  low  red  heat  increased  about  lo  times  the  solubility 
of  the  phosphoric  acid  contained  in  them  in  fifth  normal 
nitric  acid  and  rendered  it  almost  completely  soluble  in 
12  per  cent,  hydrochloric  acid.  From  these  results 
he  concludes  that  the  method  for  determining  phos- 
phoric acid  in  soils  as  used  by  him  is  not  suited  to 
distinguish  between  the  organic  and  the  inorganic 
phosphorus  in  soils,  since  the  inorganic  phosphates 
are  made  more  soluble  through  ignition. 

We  are  not  concerned  in  this  paper  with  the  latter 
conclusion  of  Fraps  as  regards  the  distinction  between 
the  organic  and  inorganic  phosphorus  of  the  soil. 
Having  frequently  noted,  however,  in  our  analytical 
work  on  soils  that  the  very  opposite  effect  to  that 
above  quoted  is  accomplished  by  ignition  of  soils 
before  determining  the  phosphoric  acid,  it  was  deemed 
desirable  to  make  some  systematic  tests  with  reference 
to  the  matter,  and  determine,   if  possible,  the  cause 

J  This  Journal,  3,  335.  

(I) 


of  the  different  effect  of  ignition  on  the  solubility 
of  phosphates  in  minerals  and  in  soils,  if  such  a  dif- 
ferent effect  was  definitely  shown  to  exist.  Accordingly 
five  different  soils  were  selected,  each  sifted  through 
a  0.5  mm.  sieve  and  prepared  for  analysis  for  phos- 
phoric acid.     The  soils  were  as  follows: 

No.  I.  A  very  rich,  black,  fine  silty  loam,  rich  in 
phosphoric  acid  and  humus,  from  Arroyo  Grande, 
San   Luis   Obispo   County,    California. 

No.  2.  A  fine  sandy  soil  with  a  good  supply  of  phos- 
phoric acid,  but  poor  in  humus,  from  Escondido, 
San  Diego  County,  California. 

No.  3.  A  very  heavy  black,  clay  adobe  soil  with 
sufficient  phosphoric  acid  for  fair  crop  production, 
from  "West   Berkeley,   Alameda   County,   California. 

No.  4.  A  sandy  loam  with  a  fairly  good  humus  and 
phosphoric  acid  content  from  South  San  Francisco, 
California. 

No.  5.  A  light  sandy  loam,  of  fairly  good  humus  con- 
tent and  phosphoric  acid,  from  Anaheim,  Orange 
County,  California. 

The  analysis  was  carried  out  as  follows:  Four 
grams  of  soil  were  placed  in  a  porcelain  cup  and  di- 
gested with  concentrated  nitric  acid  for  two  days 
on  the  steam  bath.  Two  samples  of  each  soil  were 
ignited  for  twent}'-  minutes  at  a  low  red  heat  in  a  plat- 
inum dish  and  two  others  of  each  soil  were  placed  in 
the  cups  without  ignition.  The  greatest  caution 
was  exercised  to  prevent  loss  by  dusting  in  the  process 
of  ignition  and  in  the  transfer  from  platinum  to  di- 
gestion cup.  After  the  period  of  digestion,  the  soil 
cups  were  placed  on  the  sand  bath  and  evaporated 
to  dryness  to  dehydrate  the  silica,  after  which  more  acid 
and  a  little  distilled  water  were  added  and  the  cups 
were  replaced  on  the  steam  bath  for  four  hours. 
The  solutions  were  then  carefully  filtered  and  the  soil 
residue  washed  on  the  filter.  The  filtrate  was  warmed 
to  a  temperature  of  65°  to  70°  C.  and  the  phosphoric 
acid  precipitated  with  ammonium  molybdate.  The 
precipitate  and  solution  were  allowed  to  stand  at 
65°  to  70°  C.  for  about  four  hours,  then  filtered  and 
the  precipitate  washed  with  a  solution  of  ammonium 
nitrate.  After  this  it  was  dissolved  out  with  ammonia, 
and  precipitated  with  magnesia  mixture,  then  al- 
lowed to  stand  over  night,  filtered  the  next  morning 
and  washed  with  distilled  water  containing  a  little 
ammonia.  The  precipitate  and  filter  were  then  dried, 
ignited,  weighed  and  the  phosphoric  acid  determined 

(2) 


in  the  usual  way.  The  percentages  given  represent 
averages  of  closely  agreeing  duplicates  for  each  soil 
both  ignited  and  unignited. 

Table  I. — Percentages  P2O5. 

Soil  No.                           12                  3                  4  5 

Ignited 0.410          0.150          0.090  0.167  0.151 

Not  ignited 0.460          0.250          0.130  0.191  0.190 

The  data  given  in  Table  I  leave  no  room  for  doubt 
as  to  the  effect  of  ignition  on  soil  phosphates  as  ex- 
isting in  various  soils.  In  most  cases  the  loss  in  per- 
centage is  not  very  great,  but  it  is  always  definite 
and  in  some  cases,  as  in  soil  No.  2,  it  is  quite  marked. 
In  practically  all  cases  the  duplicates  agreed  within 
one  hundredth  of  a  percent.  We  obtain  therefore 
the  very  reverse  effect  of  ignition  on  the  soil  phos- 
phates as  existing  in  the  soil,  from  that  obtained  by 
Fraps  on  mineral  phosphates  as  existing  in  minerals. 
In  brief,  ignition  of  soil  appears  to  decrease  appreci- 
ably and  definitely  the  solubility  of  its  phosphates 
whether  they  be  largely  inorganic  or  organic.  The 
large  variety  of  soils  employed  would  seem  further 
to  strengthen  the  evidence,  in  that  they  doubtless 
contained  largely  varying  proportions  of  inorganic 
and  organic  phosphates  and  a  large  variety  of  phos- 
phate bearing  minerals. 

Just  why  these  important  differences  exist  in  the 
effects  of  ignition  on  phosphates  in  minerals  and  in 
soils  is  an  explanation  that  is  not  as  easily  found 
as  the  facts  which  obtain  in  the  case.  It  would  ap- 
pear to  be  clear  that  the  increased  solubility  of  the 
phosphoric  acid  in  minerals  like  wavellite,  variscite 
and  others  tested  by  Fraps  might  be  accomplished 
through  the  mechanical  changes  induced  by  the  heat 
in  the  mineral  employed.  We  know  for  example 
that  heating  of  sand  will  disintegrate  its  grains  and 
thus  produce  more  surface.  It  is  altogether  likely 
that  the  same  happens  when  a  phosphate  mineral 
is  ignited  and  the  result  is  the  production  of  a  larger 
surface  for  the  nitric  acid  to  act  upon,  hence  an  in- 
crease in  solubility  of  phosphoric  acid  in  the  ignited 
over  the  unignited  mineral.  On  the  other  hand, 
in  a  complex  material  like  the  soil,  which  contains 
besides  many  kinds  of  mineral  and  rock  particles  in 
different  stages  of  disintegration,  very  considerable 
quantities  of  organic  matter,  both  decaying  and  in 
living  form,  and  the  minerals  and  salts  in  different 
degrees  hydrated,  it  is  probable  that  the  mineral 
particles  would  be  protected   from   disintegration   by 

(3) 


heat  so  long  as  the  water  of  hydration  and  the  organic 
matter  still  remained  in  the  soil ;  therefore  no  additional 
surface  would  be  exposed  to  the  action  of  the  acid 
and  besides,  the  dehydration  of  the  organic,  as  well 
as  inorganic  phosphates,  would  seem  to  make  them 
less  soluble.  Indeed  I  have  data  to  show  that  de- 
hydration without  ignition  also  decreases  the  solu- 
bility of  the  soil  phosphates.  This  explanation  is 
offered  by  the  writer  for  the  facts  noted,  merely  as  a 
suggestion  as  to  the  phenomena  probably  occurring 
when  mineral  phosphates  and  soils  are  heated,  as  re- 
gards their  phosphate  content.  It  is  hoped  that  an 
explanation  or  further  suggestions  will  be  forthcoming 
from  my  colleagues.  This  brief  paper  is  offered  merely 
as  a  partial  report  on  one  phase  of  the  subject  of  the 
determination  of  phosphoric  acid  in  soils,  the  whole 
of  which  is  now  being  studied  in  our  laboratory.  Other 
interesting  data  besides  that  mentioned  has  already 
been  obtained  which  we  hope  to  include  in  another 
paper  in  which  the  whole  subject  will  be  treated  more 
completely. 

Soils  Research  Laboratory, 
UNivERSiTy  OF  California,    Berkeley. 


(4) 


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